1. Technical Field
The present invention is directed generally toward a method and apparatus for improving performance in a Small Computer System Interface (SCSI) topology, and in particular, the present invention provides a method and apparatus for improving performance on a SCSI bus by pipelining SCSI nexuses to associate nexus attributes and enhance data throughput.
2. Description of the Related Art
SCSI is a hardware interface system that allows for the connection of multiple devices to a single SCSI host adapter. Typically, this host adapter is connected to a motherboard in a computer through a Peripheral Component Interconnect (PCI) slot. With this type of system, peripherals are daisy chained together. These peripherals have a second port used to connect the next device in line. This type of interface system is widely used, from desktop Personal Computers (PCs) to mainframe computers, and in Redundant Array of Independent Disks (RAID) configurations.
Information can be transferred on a SCSI bus between two SCSI devices at any given time. When two SCSI devices transfer information on a SCSI bus, one device functions as an initiator and the other as a target. An initiator initiates an Input/Output (I/O) process on a bus, and the target performs the I/O process involved. SCSI devices may be arranged as any combination of initiators and targets, but there has to be at least one of each. An initiator can also be referred to as a SCSI initiator port, and a target can also be referred to as a SCSI target port.
“Information unit” transfers on a SCSI bus are parallel transfers of information, such as, for example, data, status, commands, task attributes, task management information, nexus information, and SCSI Parallel Interface (SPI) information unit Cyclic Redundancy Check (iuCRC) information. If information units are to be transferred on a SCSI bus, an I/O process is initiated by establishing a nexus. A nexus is a relationship that exists for transferring information units between a SCSI initiator port and a SCSI target port. This relationship can be extended to a logical unit (an addressable entity within a SCSI target device) and a queue tag (parameter associated with a task for a logical unit from a SCSI initiator port).
Currently, many SCSI initiators can process only one type of data traffic at a time. For example, an initiator can establish a nexus to transfer certain data to a target. For one type of data, the initiator may assemble the data as information units into packet form. The packetized data is then transferred to the target. For another type of data, the initiator may establish another nexus and again transfer data to the target as information units, but now in non-packetized form. However, the transfer of the non-packetized data is not allowed to proceed until the initiator has completed the packetized data transfer. These queuing delays create stall conditions on the SCSI bus, which significantly reduces data throughput on the bus and thereby degrades system performance.
A similar throughput problem arises when existing initiators attempt to process SCSI bus status packets. For example, for any given nexus, a conventional initiator can process only one SCSI bus status packet at a time. This serial processing of status packets creates stall conditions on the SCSI bus, which in turn, reduces data throughput and degrades performance of the bus.
Therefore, it would be advantageous to have an improved method and apparatus for pipelining SCSI nexuses so as to associate all nexus attributes on a per nexus basis, and thereby increase data throughput and performance on a SCSI bus.